Macromolecules 2006, 39, 5159-5164
5159
Lamellar Crystal Orientations Biased by Crystallization Kinetics in Polymer Thin Films Yu Ma,† Wenbing Hu,*,† and Gu1 nter Reiter‡ Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing UniVersity, 210093, Nanjing, China, and Institut de Chimie des Surfaces et Interfaces, CNRS-UHA, 15, rue Jean Starcky, B.P. 2488, 68057 Mulhouse Cedex, France ReceiVed April 9, 2006; ReVised Manuscript ReceiVed May 18, 2006 ABSTRACT: We report dynamic Monte Carlo simulations of polymer crystallization confined in thin films of thicknesses comparable to the polymer-coil sizes. We considered two contrasting affinities of the walls to the polymers, namely sticky walls that arrest the movement of polymers in contact with the substrate (such adsorbed layers allow to avoid dewetting) and slippery walls reflecting neutral repulsion of polymers. We found that at high temperatures slippery walls slightly enhance the crystallization rate with the decrease of film thickness, and the surface-assisted crystal nucleation results in dominant edge-on lamellar crystals (chain axis parallel to the wall); on the contrary, sticky walls significantly depress the crystallization rate, and the random crystal nucleation yields preferentially flat-on lamellar crystals (chain axis normal to the wall). The growth of self-seeded crystals demonstrates that the flat-on dominance is a kinetic phenomenon due to a stronger restriction on the thickening growth of edge-on lamellar crystals.
Introduction Thin films introduce one-dimensional spatial confinement of polymers which affects their crystallization behavior.1 It is wellknown that polymer crystallization is initiated by crystal nucleation, and at high temperatures, such spatial confinement will significantly lower the formation of crystal nuclei. On the other hand, flat walls may induce a preferentially parallel orientation of the conformations of the polymers in contact with these walls. This has already been identified by both diffuse neutron-scattering experiments2 and molecular simulations.3-7 The parallel preorientations of polymer chains can compensate to some extent for the reduction in the probability of crystal nucleation and, by the way, orient the crystals with their c-axis (normal to the lamella) parallel to the confining walls. Such may lead to predominantly edge-on lamellar crystals. Thus, the interaction with the walls may provide a mechanism for controlling crystal orientations and hence properties of polymer thin films. The preferential orientations of polymer crystals in thin films have been widely studied in experiments. Roughly, experimental observations can be classified into three categories of film thickness. The first category includes those observations with films thicker than several hundred nanometers. There, predominantly edge-on lamellar crystals were found. Examples include isotactic polypropylene (iPP),8,9 multilayer coextruded iPP/ polystylene (PS),10 polyamide-6,11 polyethylene (PE),12-14 poly(ethylene oxide) (PEO),15 poly(ethylene naphthalate),16 poly(ethylene terephthalate) (PET),17 poly(-caprolactone) (PCL) in a poly(vinyl chloride) blend,18 and coarse-grained poly(vinyl alcohol) in molecular dynamics (MD) simulations.19 The second category includes those observations for films of thicknesses close to the polymer coil size (
